The invention provides a bridge strengthening system and method based on retard-bonded prestress. The strengthening system comprises a beam body. Anchor bars are implanted on the bottom surface of the beam body. A concrete layer is poured on the lower portion of the beam body and internally provided with steel bar meshes. The steel bar meshes are fixedly connected with the lower ends of the anchor bars. The strengthening system further comprises an L-shaped anchor steel plate. The anchor steel plate comprises a horizontal part and a vertical part. The horizontal part is fixed to the bottom surface of the beam body through an anchor bolt. The concrete layer is internally provided with a retard-bonded prestressed steel strand. The two ends of the retard-bonded prestressed steel strand are anchored to the outer end surface of the vertical part of the anchor steel plate. According to the bridge strengthening system and method based on the retard-bonded prestress, compared with an existing prestressed active strengthening technology, an anchorage system and a construction process are simplified, and the long-term strengthening effect is more significant; and compared with a traditional section increasing strengthening method, double anchorage of bond anchorage based on the retard-bonded prestress and the anchor steel plate at the end is adopted, the structure size of newly added concrete is decreased, and the anchorage effect of prestressed bars is improved.